The invention relates to improvements in friction clutches, especially for use in motor vehicles. More particularly, the invention relates to improvements in friction clutches wherein a pressure plate is non-rotatably connected to a rotary housing so that it can perform limited axial movements relative to the housing, and wherein a clutch disc or clutch plate can be clamped between the pressure plate and a counterpressure plate (such as a flywheel) under the action of a diaphragm spring which bears against the pressure plate in the engaged condition of the friction clutch. The diaphragm spring is axially stressed between the pressure plate and the housing and is tiltable relative to a seat which is carried by the housing. The friction clutch further employs an adjusting unit which compensates for wear upon the friction linings of the clutch disc.
Automatic compensating devices which ensure that the pressure plate is acted upon by the diaphragm spring with a substantially constant force are disclosed, for example, in published German patent applications Serial Nos. 29 16 755 and 35 18 781. These compensating devices are adjustable in response to signals from a sensor and are disposed or operate between the pressure plate and the diaphragm spring. The pressure plate is coupled to the housing by tangentially extending leaf springs which must be designed to exert a relatively small force because such force opposes the bias of the diaphragm spring. Therefore, when the friction clutch is disengaged, the pressure plate (whose mass is rather substantial) is likely to perform axial reciprocatory movements which involve repeated stray movements into and out of contact with the diaphragm spring. This not only adversely influences the operation of the friction clutch, but can actually cause the friction clutch to become a safety risk because the compensating device is being adjusted in the disengaged condition of the friction clutch until the pressure plate comes into contact with the clutch disc, i.e., when it is no longer possible to disengage the friction clutch. Therefore, compensating devices which are disclosed in the aforediscussed published German patent applications failed to gain acceptance by the makers of friction clutches.
Presently known proposals to compensate for wear upon the friction linings of a friction clutch are further disclosed, for example, in German Pat. No. 29 20 932. According to that patent an axially movable adjusting device is installed between the diaphragm spring and the pressure plate. The actual axial adjustment is or can be effected by wedges which bear against the pressure plate and can move the adjusting device toward the diaphragm spring in response to angular displacement of the adjusting device relative to the pressure plate. It is also proposed in that patent to change the positions of the wedges for the purpose of moving the adjusting device relative to the pressure plate.
The means for ascertaining the extent of wear upon the friction linings in conventional friction clutches of the above-outlined character comprises a plurality of sensors which operate between the pressure plate and the counterpressure plate (normally a flywheel) or between the pressure plate and the cover. When the clutch is engaged, the sensors are displaced by a distance depending on the extent of wear upon the friction linings, and the extent of movement of the pressure plate away from the counterpressure plate in response to disengagement of the clutch is altered depending on the extent of adjustment of the sensors. The operation of conventional automatic wear compensating systems is based on the assumption that, when the friction clutch is new, the disengagement involves a movement of the pressure plate (away from the counterpressure plate) which exactly matches the extent of movement of the diaphragm spring. When the friction linings undergo a certain amount of wear, the pressure plate moves toward the counterpressure plate (flywheel) through a distance which is dependent on the extent of wear. Therefore, in the absence of any undertakings to the contrary, the diaphragm spring is then required to cover a greater distance from the cover toward the counterpressure plate in order to bias the pressure plate against the adjacent friction linings, i.e., to clamp the friction linings of the clutch disc between the pressure plate and the counterpressure plate. In other words, the distance which is covered by the diaphragm spring then exceeds the distance covered by the pressure plate. This results in appropriate axial displacement of the adjusting device in a direction toward the diaphragm spring.
A drawback of the conventional proposals is that the adjustment does not always correspond to the extent of wear upon the friction linings. The reason is that the extent of movement of the diaphragm spring in response to engagement or disengagement of the friction clutch fluctuates within a certain range, even if the extent of movement to disengage or engage the friction clutch is constant. The bearing or bearings which form part of the disengaging means and serve to displace the customary prongs of the diaphragm spring during disengagement of the friction clutch are particularly likely to perform movements which depart from the expected or prescribed movements within a rather wide range. Furthermore, when the clutch disengaging mechanism (e.g., a mechanical disengaging system) is adjusted, its actual adjustment is likely to depart from an optimal adjustment, for example, because the clearance between the disengaging system and the diaphragm spring is too small and/or because the initially selected stress between the disengaging system and the diaphragm spring is too pronounced. This results in the establishment of operating points which depart from the optimal operating point of the friction clutch.
If a friction clutch is equipped with an automatically adjustable disengaging system e.g., with a hydraulic system which employs a master cylinder and a slave cylinder, the aforediscussed means which is to compensate for wear upon the friction linings cannot be operated at all because the extent of movement to disengage the friction clutch is the same during the entire useful life of the clutch. Thus, no adjustment will take place as long as the movement of the diaphragm spring in the region of the adjusting device during disengagement of the friction clutch is less than or at most matches the extent of movement of the pressure plate. On the other hand, if the extent of movement of the diaphragm spring exceeds the extent of movement of the pressure plate, the adjusting device is actuated and carries out an adjustment during each disengagement of the friction clutch irrespective of the extent of wear (or any wear) upon the friction linings so that the friction clutch is totally misadjusted after a relatively small number of disengagements.
An additional problem which arises in conventional self-adjusting friction clutches (i.e., in clutches wherein the relative positions of the pressure plate and the diaphragm spring are to be automatically changed as the wear upon the friction linings progresses) is that the pressure plate is likely to become disengaged from the diaphragm spring in the axial direction of the clutch in response to characteristic resonant vibrations. This results in the activation of the adjusting device which induces a total misadjustment of the friction clutch and renders it useless for its intended purpose.
Friction clutches of the above-outlined character are further disclosed in published German patent application Serial No. 24 60 963, in German Pat. No. 24 41 141, in German Pat. No. 898 531 and in German Auslegeschrift No. 1 267 916.
A clutch assembly has a friction clutch comprising a pressure plate which is non-rotatably connectable with a counterpressure plate for limited axial movement with respect thereto, and at least one biasing spring provided to urge the pressure plate toward a clutch disc which can be clamped between the pressure plate and the counterpressure plate. There is further provided an adjusting device to compensate at least for the wear upon the friction linings of the clutch disc so as to effect a substantially unchanged application of force by the biasing spring to the pressure plate. The friction clutch further comprises actuating means for engaging and disengaging the clutch. The actuating means is operable by a disengaging member which is shiftable axially by a disengaging means, e.g., a disengaging fork which is pivotably mounted on a transmission case.
A clutch assembly which is constructed and which is operable in the aforedescribed manner is known, for example, from the published French patent application No. 2 582 363. The actuating means of such clutch assemblies can be acted upon by disengaging systems, or by disengaging means and a disengaging member in a manner as disclosed, for example, in U.S. Pat. No. 4,368,810, U.S. Pat. No. 4,326,617, the published German patent application No. 27 52 904 and the published German patent application No. 27 01 999.
Clutch assemblies or friction clutches utilizing an integrated adjusting device which compensates at least for the wear upon the friction linings of the clutch disc exhibit the drawback that, especially when employing so-called mechanical disengaging systems wherein the movements of the clutch pedal are transmitted to the actuating means of the friction clutch by way of a linkage and/or a Bowden wire with the interposition of at least one disengaging bearing, due to tolerances within the entire kinematic train, one cannot ensure that those portions of the disengaging member which act upon the actuating means invariably assume the same axial positions relative to those portions of the actuating means which are to be acted upon. This can result in relatively large departures of the disengaging path of the friction clutch or of the extent of actuating movement which is being imparted to the actuating means from an optimum departure or from an optimum extend of actuating movement. At the very least, an excessive a departure can adversely influence the operation of the adjusting device to such an extent that, under certain extreme circumstances, the adjusting device is no longer capable of carrying out its adjusting function. Furthermore, it can happen that, under certain circumstances when the actuating means covers an excessive distance resulting in undesired adjustment, the friction clutch is no longer capable of being properly disengaged or the initial stressing and the position of the biasing spring are altered to such extent that the force which is actually supplied by the biasing spring is not suffice to guarantee an acceptable transmission of torque.
Published German patent application Serial No. 40 92 383 discloses means for automatically adjusting the position of the pressure plate relative to the counterpressure plate in response to wear upon the friction linings of a clutch disc and (if necessary) in response to wear upon certain other parts, such as the friction surfaces of the two plates. The purpose of the automatic adjustment is to ensure that the bias or clamping action of the friction surfaces on the pressure plate and on the counterpressure plate upon the adjacent friction linings of the clutch disc remains unchanged irrespective of the extent of wear of the friction linings and the friction surfaces of the two plates. The German patent application proposes the utilization of a wear compensating system which employs two annular members and is designed in such a way that the annular members can move axially in response to wear of the friction linings. Such axial adjustment of the annular members determines the axial position of the location(s) of engagement between the diaphragm spring and the pressure plate and thus compensates for wear, at least of the friction linings. Thus, the position of the diaphragm spring should remain unchanged (namely the same as in a new friction clutch) regardless of the extent of wear upon the friction linings.
A drawback of the friction clutch which is disclosed in the published German patent application No. 40 92 383 is that the radial distance of the contact points on the diaphragm spring between the two annular members (one of which surrounds the other) is too small. Moreover, the friction clutch of the German patent application does not allow for accurate initial or subsequent adjustment of the pressure plate, partly due to matching tolerances (especially as concerns the thickness of the annular members in the axial direction of the pressure plate) and partly due to progressive wear of the diaphragm spring and its seat in the friction clutch. A pronounced increase of the distance between the inner and outer annular members is not possible because this would greatly reduce the extent of mobility of the pressure plate in response to the engagement or disengagement of the friction clutch, i.e., the distance of the pressure plate from the counterpressure plate in the engaged and disengaged conditions of the friction clutch would be too small. An undue reduction of such distance would affect the operability of the friction clutch, i.e., attempted disengagement of the friction clutch would not result in an interruption of transmission of torque between the pressure plate and the counterpressure plate on the one hand, and the friction linings of the clutch disc on the other hand.
An additional drawback of the friction clutch which is disclosed in the German patent application No. 40 92 383 is that the pressure plate (which is normally affixed to the housing of the friction clutch by leaf springs serving to permit axial movements, but to prevent rotation, of the pressure plate relative to the housing and relative to the counterpressure plate) is free to oscillate in the axial direction when the friction clutch is disengaged. This can result in undesirable adjustment of the annular members relative to the pressure plate in a direction toward the diaphragm spring while the pressure plate is in the process of moving axially and away from the diaphragm spring. Upon the next-following engagement of the friction clutch, the diaphragm spring is likely to assume an improper position (namely a position corresponding to a partially disengaged condition of the friction clutch) with the result that the bias of the diaphragm spring upon the pressure plate in the engaged condition of the friction clutch would depart from the bias during preceding engagement. This would prevent full disengagement of the friction clutch when the operator desires to interrupt the transmission of torque between the pressure plate and the counterpressure plate on the one hand, and the clutch disc (i.e., the friction linings) on the other hand.
Additional prior proposals to automatically compensate for wear upon certain parts, particularly upon the friction linings, of friction clutches are disclosed, for example, in published German patent applications Serial Nos. P42 39 291.8, P43 06 505.8, P42 39 289.6, P42 31 131.4, P42 43 567.6 and P43 17 587.2. The disclosures of all enumerated U.S. patents German patent applications are incorporated herein by reference.
The German patent applications which are enumerated in the preceding paragraph disclose automatically self-adjusting friction clutches which are constructed and assembled in such a way that the pressure plate is urged against the adjacent friction linings of the clutch disc with a pronounced force, but the disengaging force remains low. Moreover, the disengaging force should remain constant or nearly constant during the entire useful life of the clutch. Otherwise stated, the disengaging force should remain at least substantially constant prior to any wear upon the friction linings as well as while the friction linings undergo wear which is to be automatically compensated for until the wear upon the friction linings is too pronounced to warrant further compensation so that the friction clutch must be discarded. This is proposed to be accomplished by employing a clutch spring (such as a diaphragm spring) whose characteristic curve is very steep, i.e., which should be capable of abruptly or rapidly reducing its bias upon the pressure plate. During disengagement of the friction clutch, the disengaging force should fluctuate very little or not at all. This, in turn, fails to cause the distance-to-force progress of the characteristic curve of the clutch spring to be available at all times so as to ensure a predictable and complete disengagement of the friction clutch with at least some spare distances to be covered in order to compensate for eventual tolerances. The primary reason is believed to be that the characteristic curve of a clutch spring (diaphragm spring) having a steep characteristic curve during disengagement of the clutch includes a pronounced upwardly sloping portion which immediately follows the downwardly sloping portion. This can be seen in the enclosed FIG. 94 wherein the fluctuations of the force of a clutch spring are measured along the abscissa and the distance which is covered by the spring during dissipation of energy is measured along the ordinate. When the clutch employing a diaphragm spring having a characteristic curve of the type shown in FIG. 94 by a solid line is engaged, the distance which is covered by the spring is at least close to 1 mm. The distance is approximately 2 mm when the pressure plate of the clutch starts to move away from contact with the adjacent friction linings, and the characteristic curve exhibits a rather pronounced upward slope when the covered distance approximates or equals 3 mm. That point of the curve which is reached when the covered distance equals or approximates 3 mm corresponds to the required minimal distance to be covered during disengagement of the friction clutch. This does not take into consideration any of those tolerances which invariably develop during disengagement of the clutch, tolerances developing during assembly of the friction clutch, machining tolerances of component parts of the friction clutch, as well as losses of elasticity during the useful life of the clutch. Additional fluctuations of the distance to be covered by the clutch spring develop due to tolerances of the disengaging means so that the required minimum distance to be covered during disengagement of the clutch is normally at least 3.5 mm. As can be seen in FIG. 94, this distance is covered while the bias of the clutch spring is already on the increase, i.e., when the disengaging force is again quite pronounced. If the clutch employs a sensor of the type disclosed in the aforementioned German patent application Serial No. P 43 39 291.8, the adjusting ring of the wear compensating unit is likely to change its angular position and to thus compensate for non-existent wear or to compensate before the friction linings have undergone sufficient additional wear to warrant a renewed compensation.
An additional drawback of many heretofore known proposals to construct, assemble and operate friction clutches which are equipped with automatic wear compensating means is that the bearings, levers and/or other parts which are used to furnish the disengaging force must be designed to transmit large or extremely large disengaging forces. This, in turn, necessitates the provision of large, complex and expensive thrust bearings for the output element of the prime mover (e.g., the crankshaft or the camshaft of a combustion engine in a motor vehicle) in order to ensure that the bearings will withstand those stresses which develop as a result of the application of pronounced disengaging forces.